This invention relates generally to electroplating apparatus and methodology, and more specifically relates to the electroplating with gold of electronic components or the like.
Gold, within recent years, has become a very important part of the electronics industry. Among those properties recommending its use therein, are its relative unalterability, high solderability and low contact resistance. In the semi-conductor field, gold has furthermore found favor because of its ability to readily form an eutectic alloy with silicon and germanium. In the latter connection it may be noted that most headers or packages for diodes, transistors, and integrated circuits are gold plated as a preparation for the mounting or attaching of semi-conductor devices. Such components are exemplified by the well-known line of TO-5 and TO-8 multi-lead headers. Such headers consist of an eyelet of Kovar metal to which several insulated Kovar leads are attached, and sealed in glass.
In accordance with known principles in the art, headers of the foregoing type have, in the past, been plated (among other methods) by so-called barrel plating techniques -- that is, by subjecting such articles to electroplating while a plurality of articles tumble in a barrel. These barrel techniques, however, have many important drawbacks, numerous of which are recognized in the art. For example, where headers or the like are thus plated, it is found that many leads do not make electrical contact with the remainder of the load. Where such conditions obtain during the plating cycle, the portion of the lead closest to the anode becomes cathodic. Such leads become bipolar, and at the anodic portion of the leads problems can arise in that the gold may redissolve anodically, and as well base metal can be attacked to expose bare spots. Where the tumbling action is markedly inadequate these problems can become quite severe. In the past these problems have partially been overcome by incorporating mechanical means for improving electrical conductivity through the load. Such means have taken the form of metal particles or metal shot. Unfortunately during plating operations the shot itself becomes gold plated, resulting in the loss of gold and attendant increase in the cost of plating the desired objects, that is, the headers, etc.
Within recent years, particularly because of the soaring price of gold, it has furthermore been increasingly appreciated that barrel plating techniques (and as well, common rack plating techniques) are exceedingly wasteful of the gold itself. If one considers, for example, the most common use of barrel plating in the electronic industry, i.e. the plating of the aforementioned headers, it will be appreciated that basically one is only interested in providing a plating at the die-receiving face thereof, and at the contact connections for the header leads which are present at the said face. Barrel plating techniques, however, are such that the entire header is plated with gold -- including all electrically conductive, accessible portions thereof. Furthermore, since barrel plating is based upon the development of multiple electrical contacts among the tumbling components, it is basically a statistical process, this is to say that different components in a tumbled load may be subjected to markedly different plating times. In order to achieve a desired mean plating thickness, it is therefore necessary to grossly overplate. In order to assure that all of the individual components in the batch receive adequate plating, it is frequently necessary to overplate many of the components by as much as 10% to 20%. This is obviously a further waste of the precious gold material.
In U.S. Pat. No. 3,904,489 to Frank J. Johnson, which patent is assigned to the same assignee as is the instant application, apparatus and methods are disclosed which are highly effective in overcoming the foregoing problems. In such Johnson apparatus the components to be selectively plated are conveyed across the surface of a moving electroplating applicator with the portions selected to be plated in contact with the electroplating solution. A DC electrical potential is applied between the portions of the components which are to be plated and the back of the applicator surface, to enable the electroplating action.
It may be noted in the foregoing connection, that a peculiar and specific problem that is present where components of the type discussed are subjected to the described selective electroplating operations, arises because the components are possessed of not only a relatively flat die-receiving face, but the face as mentioned, is provided with a plurality of electrical contacts which are insulated from the remainder of the face, and are electrically accessible (during plating) primarily from wire like leads which extend oppositely from the face. The peculiar problem that is thus presented, is that a potential must be applied not only to the body of the component, i.e. so that the die-receiving face may be suitably electrified, but moreover a potential must be enabled to each of the insulated contacts -- if one desires to plate same.
Improvements upon the aforementioned Johnson apparatus are disclosed in a copending application of Maurice Bick et al, Ser. No. 472,952, filed May 31, 1974, and entitled "Selective Plating Apparatus", this application also being assigned to the assignee of the present application. In the cited application the improved apparatus is characterized by an arrangement wherein the conveyor belt for the components passes through a channel in a stationary guide means at the electroplating station, which guide means accurately spaces the components with respect to the applicator, and restrains the components from undesired wobble or vertical movements. The leads of the components, as mentioned, are connected to the electrically isolated terminals or contacts on the die-receiving face of the components, and such leads protrude from the guide as they progress through the channel therein. Electrical contact with the leads, for purposes of plating the isolated contacts, is made by a flexible conductor -- which may be a brush or similar conductive surface, which can be maintained stationary as the components are swept past same with the leads in contact with the surface.
It has been found in use of the various aforementioned selective plating apparatus that the plating quality achieved on the isolated contacts, particularly vis-a-vis the plating at the die-receiving face, can exhibit unacceptable aspects. A component emerging from such prior art type device may thus appear perfectly satisfactory to the observer; but upon being subjected to the standard bake-out tests utilized in the semi-conductor industry, the die-receiving face may exhibit satisfactory plating, while the contacts are unacceptably plated -- in that cracking or so forth may occur. In order to render the plating completely satisfactory in all respects, it has been thought that the overall plating must be increased to a point whereat relatively high thicknesses are provided for the contacts. This in turn is wasteful of the gold, in that unnecessary and inordinate amounts are plated upon the die-receiving faces.